Transparent glass or plastic substrates (e.g., optical substrates) can experience a substantial loss of optical performance due to unwanted reflections from an air-substrate interface. When multiple interfaces are present within a display, the loss of viewing efficiency can be large. The losses from the air-substrate interface can be described by the Fresnel equation:((nd−1)/(nd+1))2*100=% reflectionIn this equation nd represents the refractive index of the optical substrate and 1 represents the approximate refractive index of air.
This loss of optical performance becomes apparent when one tries to view an image or text through a transparent glass or plastic substrate. In conditions of high ambient lighting, the surface reflection becomes so intense that one cannot readily view the text or the images through the transparent substrate.
Various solutions to this problem have been disclosed in the prior art. The most common of these is to coat the transparent substrate with a layer of material, which has a refractive index lower than that of the substrate and that has an optical thickness of approximately one-quarter the wavelength of the light of interest. For instance, by coating a poly(ethylene terephtalate) (PET) film with a single layer (e.g., about 0.100 microns thickness) of gas phase deposited SiO2, the percent reflectance can be decreased from about 5.75 percent per side to about 1.50 percent per side, with concomitant improvements in viewing efficiency. As the number of functional layers increases, the efficiency of these coatings also improves dramatically, that is, these coatings go from being quite narrow in their performance characteristics to quite broad, as one goes from 2-layers to greater than 3-layers in an optical stack.
U.S. Pat. No. 6,245,428 assigned to CPFilms, Inc., discloses an anti-reflective coating that utilizes in a low refractive index fluorine-containing polymer in conjunction with a high refractive index organic-inorganic composite. The high refractive index organic-inorganic composite comprises a titanium (IV) ester reacted with organo-silicone compounds to produce a ceramer composition that exhibits high stability in conjunction with a relatively high refractive index. The ceramer is combined with inorganic oxides, such as iron oxide, and is capable of producing a refractive index of about 1.60 by dispersion of the oxide particles in the ceramer composition.
U.S. patent application Ser. No. 20020119304 assigned to 3M Innovative Properties discloses a process for manufacturing high refractive index colloidal oxide particles for modification of a polymer's properties. Also disclosed are anti-reflective coatings manufactured using these particles as a means to modify the properties of the resulting high refractive index layer. The refractive index for a polymer organic matrix with and without colloidal oxide modification were, respectively, 1.65 and 1.46. As a result of the relatively low refractive index of the organic matrix, very high levels of colloidal oxide must be used to obtain layers that have refractive indices high enough to be useful in anti-reflective coatings.
The invention disclosed in this application and in U.S. Pat. No. 6,245,428 suffer from these constraints due to limited availability of UV curable monomers in the proper refractive index range. Choices of available UV curable monomers are limited to a refractive index range between 1.48 and 1.56. These ranges are less than optimal, as the refractive index of a blend is approximately the weighted volume percent averages of the respective refractive indices of the component materials. Thus, if the UV curable matrix starts with a lower refractive index, then the valuable properties of the high refractive index particles are unnecessarily reduced by the addition of the UV curable matrix. In addition, it is also difficult, in practice, to prepare coatings that contain enough high index nano-particles to overcome this specific limitation.
U.S. Pat. No. 5,991,081 assigned to Peter D. Haaland, discloses the preparation of anti-reflective coated lenses by the evaporation of fluoropolymers in a vacuum. Due to use of a vacuum, the size of the substrate that can coated is limited by sizes of available vacuum chambers. Additionally, as the size of the vacuum chamber increases, the costs, the cycle time and the loss of economic efficiency also increase.
U.S. Pat. No. 5,925,438 assigned to DaiNippon Printing Company, teaches using sol-gel chemistry to produce an alternating stack of high and low refractive index layers. The low refractive index layer is prepared by hydrolysis of trialkoxymethylsilanes. The high refractive index layer is prepared via hydrolysis of titanium (IV) esters to form an organo-titanium compound.
U.S. Pat. No. 5,856,018 teaches a dip coating process for manufacturing anti-reflective coated sheets. Particular coating compositions used in this process are comprise starting monomers, such as titanium (IV) esters and tetraethyl ortho silicates. Using these monomers, anti-reflective coatings are prepared with less than 0.30 percent reflectance per side, high durability and broadband performance.
Asahi Glass and DuPont (DuPont Company literature: Teflon® AF brochure and Asahi Glass Company technical information bulletin: Amorphous fluoropolymers Cytop®) have developed commercially available fluorinated polymers with very low refractive indices that are used to prepare coated films and articles that exhibit very low surface reflectance. These polymers exhibit the lowest refractive indices of any known polymers and thus can be used to prepare very simple yet effective coatings that have virtually no reflection across the visible spectrum. A specific limitation of these polymers is that they are extremely expensive and are only soluble in highly fluorinated solvents, which contributes to the expense of the resulting anti-reflective films.